Next generation mobile devices implement a plurality of wireless technologies to access different networks such as WiMAX networks, WLAN networks, LTE networks, Wireless USB or Bluetooth (BT) networks, etc. Such devices are referred to herein as “combo” devices. While increased access to these technologies benefit users and operators alike, interference among different technologies, particularly onboard a single combo device, introduces difficulties during concurrent operation of these technologies. For example, and as illustrated in FIG. 1, WLAN (in 2.4-2.5 GHz) and WiMAX (2.3-2.4 GHz and 2.5-2.7 GHz) technologies operate at relatively close frequency bands with respect to each other—so close, in fact, that the out-of-band emission by either technology may saturate the receiver of the other technology resulting in potential blocking. Thus, the interference between different technologies operating in the same combo device creates coexistence problems.
Time multiplexed operation has been proposed coordinate BT radio and WLAN radio in a single mobile device (co-existence node). Under such operation, the CTS2Self mechanism may be used to protect both BT and WLAN performance in order to avoid the avalanche effect (TI Connectivity Solutions: “WiMAX/WLAN and BT coexistence”, 2007). The protection mechanism using CTS2Self frames, however, could greatly reduce the channel utilization of WLAN, as a CTS2Self frame disables transmissions from all WLAN neighbors during the following BT activity. For example, if the BT radio has HV3 traffic and a co-existence node generates a CTS2Self frame once every 3.75 ms, the resulting channel utilization is less than 67% because transmissions from neighbors are disabled for at least 1.25 ms. Channel utilization could be worse when CTS2Self based protection is used by multiple mobile devices associated with the same AP.
In order to reduce the number of CTS2Self frames generated while avoiding the avalanche effect, a scheme that takes advantage of Power Save (PS) mode has been proposed. FIG. 2 shows this scheme in which a co-existence node (STA) stays in PS mode so that WLAN Access Point (AP) cannot transmit a data packet to the STA without having received a PS-Poll frame from the STA first. As shown in FIG. 2, after the STA has received a beacon indicating a pending data to the STA at the AP, the STA transmits a PS-Poll to notify the AP that it is active to receive the data. Upon receiving this PS-Poll, the AP replies with an ACK after a SIFS delay. Then the data is sent at AP's convenience and the STA confirms a successful receipt with an ACK. Although the 802.11 standard allows an AP to reply to the PS-Poll with a data instead of the ACK as in FIG. 2 (802.11 Spec), most products take the approach shown in FIG. 2 for better protection of the data transmission and lower complexity in implementation. Since the AP cannot transmit any data before receiving a PS-Poll from the STA, no CTS2Self frame is needed and the avalanche effect is avoided.
In WLAN networks, the AP response time and adaptive rate control can make PS-Poll-based transmissions less efficient. Most deployed APs cannot start data transmission immediately after sending an ACK in response to the received PS-Poll packet. There is often a delay of several hundreds of microseconds before an AP starts the transmission, and this depends on implementation of the AP and traffic loads at the AP. Such delays are herein referred to as “AP response delay”. Another important factor for PS-Poll efficiency is adaptive rate control at the AP. A lower PHY data rate used for data transmission results in longer data transmission time. Such long transmission times, in addition to the AP response delay, make it harder for a PS-Poll-ACK-Data-ACK sequence to be completed between two consecutive BT activities. FIG. 3 illustrates a scenario in which the AP response delay and long data transmission times result in data transmission failure. As shown in FIG. 3, a STA has no time to respond with an ACK before the STA grants the medium to the BT radio, and thus data retransmissions from the AP that overlap with BT activity will fail.
CTS2Self can be used to delay a data transmission so that it is not interrupted by BT activities (see FIG. 4). This gain, however, is at the cost of poor channel utilization. In order to delay the data transmission from the AP, a CTS2Self has to be sent before the data transmission starts from the AP. Since the size and arrival time of the data frame are unpredictable, the latest time the CTS2Self should be transmitted is a calculable time (e.g., TMaxData+SIFS+TACK) before the medium is granted to BT. Here TMaxData is the duration to transmit a data packet of maximum size and TACK is the duration to transmit an ACK. As an example, TMaxData for a data packet of 1500 Bytes may be more than 2 ms at the data transmission rate of 6M bps. Further, with a 1.2 ms time period of BT activity, the CTS2Self needs to extend the network allocation vector (NAV) at the AP by more than 3.2 ms. All STAs in the WLAN overhearing this CTS2Self will set their NAV accordingly and refrain from transmitting during this time. In this scenario, the network's channel utilization will be greatly reduced if such CTS2Self frames are transmitted often.